Elements of Barkhausen material--also known as pulse wires--are used both for electronically securing articles and for so-called pulse wire sensors in sensor technology. They are distinguished by a virtually rectangular hysteresis curve; that is, the reversal of magnetization in an outer alternating magnetic field takes place virtually abruptly as soon as the exciter field has reached the predetermined threshold value. Elements suitable for electronically securing articles are described for instance in U.S. Pat. Nos. 4,247,601 and 4,660,025 and European Patent Disclosures EP 0 309 679 B1 and EP 0 762 354 A1.
The abrupt reversal of magnetization of wirelike or striplike Barkhausen materials in an external alternating field is the consequence of a pronounced monoaxial anisotropy along the longitudinal axis of the wire or strip. This anisotropy suppresses not only disruptive rotational processes during the magnetization reversal but also the formation of terminal domain structures before the switching field intensity is attained. After all, the formation of such structures would cause a rounding off of the hysteresis curve and thus would worsen the switching characteristics.
The anisotropy required to form a markedly rectangular hysteresis curve can have various causes. Methods have become known from the patent literature that utilize voltage-induced anisotropy (U.S. Pat. No. 4,660,025) or magnetic field-induced anisotropy (EP 0 762 354 A1). However, it is common to all these methods that to support the induced anisotropies, a pronounced formal anisotropy, in order to attain the pronounced switching behavior is necessary. Shortening the strips or wires, which as a rule are elongated, causes a decrease in the formal anisotropy and an increase in the demagnetization effect, which reduces the rectangular shape of the hysteresis curve.
Increasing the intrinsic anisotropy, as proposed for instance in EP 0 762 354 A1 for labels of short length, would indeed at least partly compensate for the decrease in formal anisotropy or the increase in the demagnetization factor; however, at the same time it would lead to an increase in the switching field intensity, which is undesired for the application in systems for electronically securing articles.
From the patent literature, alternative ways have already become known for how the demagnetizing field in the end regions of the Barkhausen material, which after all causes the undesired magnetization reversal processes, can be suppressed. For instance, in European Patent Disclosure EP 0 710 923 A2, a magnetic marking element is described, which comprises a thin magnetic wire of Barkhausen material, in the two end regions of which two soft magnetic chips are disposed that have a lower coercive force than the Barkhausen material wire. To increase the pulse height, the chips cover the ends of the strips and preferably protrude past the ends to all sides.
From EP 0 762 354 A1, it has become known to use an amorphous band or an amorphous wire of Barkhausen material as a securing element for electronically securing articles. To increase the pulse height, at least one further strip of soft magnetic material is associated with the amorphous material and preferably protrudes past the ends of the amorphous material.
Both of these versions in the prior art have the disadvantage of being unsuitable for continuous production of bands from which labels of varying length are later stamped out. The reason for this is that the soft magnetic strips must be positioned at the ends of the labels, yet as a rule the position cannot be determined during production.